"It is something that has been worrying him for awhile, but it was the way he reacted after the Dragons game when we knew there was something wrong," Ponissi said.
Queenslander Cameron Munster will replace Slater at fullback for Melbourne.
Munster is fast becoming an NRL star, but there is no doubt Slater is a huge loss for Storm's premiership hopes.
"We're devastated for Bill," Ponissi said.
"We were that excited at how he was training and he was so fit.
"He's the main one I feel sorry for, but it is obviously also a massive blow for us.
"But to just have to pick ourselves up and dust ourselves up because we have a pretty resilient group here and they have a job to do."
Storm centre Will Chambers said the club felt deeply for Slater.
"It's disappointing for everyone else around the club as well," Chambers said.
"Bill is a big part of this club and has been for a long time.
"It's very disappointing and everyone has their fingers crossed for some good news over the next couple of days.
"It's obviously devastating for him.
"The news that he had to go back in and had to find out what's wrong with his shoulder is obviously hard."
Storm will next play New Zealand Warriors in Auckland on Sunday.
- news.com.au